A new betaine, halocynine, was isolated from the muscle of ascidian Halocynthia roretzi and its structure was elucidated to be (R)-2-hydroxy-6-trimethylammoniohexanoate. A variety of betaines has been found in marine invertebrates. 2-4 In the course of our continuing studies on the nitrogenous compounds in the muscle extract of ascidian H. roretzi, 1 we have encountered a Dragendorff-positive compound, whose TLC properties implied its novelty. The isolation and structural elucidation of the metabolite named halocynine (1) are the subject of this paper. Halocynine is a 2-hydroxylated analogue of laminine (2)5 which is occasionally found in the algal extracts or in some proteins. 6-9